Polarized far-infrared emission from the core and envelope of the sagittarius b2 molecular cloud

We have detected linear polarization in the 115 μm continuum radiation from the giant molecular cloud Sagittarius B2. We found polarization at nine positions in the dense cloud core and at 15 positions in the less-dense envelope. The polarization in the core is due to absorption by magnetically aligned grains and that in the envelope is due to emission from magnetically aligned grains. The inferred magnetic field direction is roughly north-south everywhere, but with spatially smooth variations of up to 30°. By considering our data together with Zeeman splitting observations we are able to set a conservative lower limit of 150 μG on the strength of the large-scale field in the envelope. If large-scale fields this strong are common in Galactic center clouds, they could be detectable via large-beam Zeeman measurements. For positions in the envelope that are furthest from the core, the field is nearly parallel to the plane of the Galaxy. This is consistent with the idea of a globally azimuthal magnetic field in the Galactic center neutral gas layer, which is expected if gravitational forces dominate magnetic forces.